1. Field of the Invention
The present invention relates to an HC adsorbent for an internal combustion engine, and particularly to an HC adsorbent mounted in one of a pair of exhaust passages which are provided in parallel to each other in an exhaust pipe downstream of an exhaust emission control catalyst, and through which an exhaust gas is selectively permitted to flow by a switchover valve.
2. Description of the Related Art
A conventional HC adsorbent suffers from the following problem: At the cold start of an internal combustion engine, an exhaust emission control catalyst in an exhaust system does not yet rise to an activating temperature, notwithstanding that an unburned hydrocarbon (which will be referred simply to as HC herein) is contained in a large amount in an exhaust gas. For this reason, the unburned HC cannot be oxidized and converted effectively by the catalyst.
To solve this problem, it has been already proposed (for example, see Japanese Patent Application Laid-open No.10-153, 112) to use the above-described HC adsorbent in combination with an exhaust emission control catalyst. More specifically, in the proposed system, the releasing of the unburned HC to the atmosphere is inhibited by shifting the switchover valve to a starting position in an inactivated state of the catalyst at the start of the engine to permit the exhaust gas to flow through the adsorbent, thereby adsorbing an unburned HC in an exhaust gas to the adsorbent. After activation of the catalyst, the switchover valve is shifted to a usual position to permit the exhaust gas to flow, bypassing the adsorbent, and the adsorbed HC is desorbed from the adsorbent. The desorbed HC is recirculated to a location upstream of the catalyst or the intake system in the internal combustion engine, whereby it is converted or reburned. In this manner, the emission in the exhaust gas can be reduced.
It has been also proposed to use a zeolite as an HC adsorbent such as an aluminosilicate, metallosilicate and the like in the conventional system with the heat resistance taken into consideration.
The number of types of hydrocarbons (HCs) contained in the exhaust gas is 200 or more, and the shapes and sizes of the HCs are various. Therefore, various types of zeolites having pores of different pore sizes such as aluminosilicate and metallosilicate zeolites, e.g., chabazite-type, ferrierite-type, pentasil-type, mordenite-type, faujasite-type, beta-type zeolites are combined in various ratios for use as an adsorbent, so that the adsorption performance for HCs having various molecular diameters is satisfied.
However, such technique suffers from a problem that the adsorption performance is enhanced for various HC, but a temperature at which the desorption of HCs from the adsorbent is completed (which will be referred simply to as a desorption temperature hereinafter) is increased.
In a structure designed so that only when a catalyst is inactivated, a switchover valve permits an exhaust gas (its temperature at this time is relatively low) to flow through an adsorbent, and after activation of the catalyst, the switchover valve permits the exhaust gas to flow through a bypass passage without flowing through the adsorbent, as in the above proposed system, the temperature of the adsorbent itself is difficult to rise and for this reason, an attempt is made to raise the temperature of the adsorbent by placing the adsorbent in the proximity to the exhaust gas bypass passage or by another means. In this case, however, there is a problem that the type of HC requiring a high temperature for desorption thereof cannot be desorbed quickly.
If the adsorbed HC is not desorbed quickly, as described above, there is a possibility that the undesorbed HC remains not a little, when the vehicle is moved to travel for only an extremely short time after the starting of the engine. In such a case, there is a possibility that the adsorbing performance of the adsorbent is reduced due to the undesorbed HC still remaining in the adsorbent at the next starting of the engine, whereby the exhaust gas purifying performance is detracted.